An example of the conventional mount structure of an optical semiconductor device is reported in K. Yamauchi et al., "A Surface Mountable Optical Module for Subscriber Network", Proceedings of the 1996 IEICE General Conference, Electronics-1, SC-2-7, pp.442-443(1996). In this example, an optical semiconductor element (PD element) and a semiconductor element (preamplifier) are mounted as pair chips so as to provide the minimization and to enhance the function and productivity.
However, in the conventional optical semiconductor device, the substrate to mount the optical semiconductor element is mounted being separated from the semiconductor element substrate to drive the optical semiconductor element. Therefore, a clearance to insert a mounting jig when mounting the substrates is required, thereby increasing the entire size of the optical semiconductor device. Also, the bonding wire to connect between the substrates must be lengthened that much, thereby increasing the wire inductance and thus preventing the device from operating at a higher speed. Further, due to the separated structure, the number of parts must be increased and the productivity must be lowered.